Atmospheric Pressure and other State Variables
- Pressure
- Force exerted by random molecular collisions against surfaces
- Units of pressure are Force/Area: Pascals = Newtons/m2
- Common units in U.S. are millibars: 1mb = 100Pa
- Also units of Energy/Volume: (NewtonXm)/m3
- Also units of Momentum Flux: (massXvelocity)/s/m2
- Atmospheric Pressure
- Force exerted by total atmospheric mass being accelerated downward
- Always must decrease with increasing height
- Rate of decrease at a given height proportional to pressure at that height
- Exponential decrease in pressure with height
- State Variables
- Pressure (p), temperature (T), and volume (V) or density
(
) - Density equals number of molecules X molecular mass / volume
- Temperature proportional to average molecular kinetic energy
- Pressure proportional to total molecular kinetic energy per volume
- Total number of molecules X average molecular kinetic energy / volume
- Relationships between state variables
- Pressure increases with increasing temperature at constant volume
- Higher temperature means larger average molecular kinetic energy
- Pressure increases with decreasing volume (increasing density) at constant temperature
- Increasing density means more molecules
- Boyle's Law
- Volume increases (density decreases) with increasing temperature at constant pressure
- Increasing temperature and larger average molecular kinetic energy produce expansion
- Gay-Lussac Law
- Equation of State
- Relates all three state variables
- Simple form for ideal gases like atmosphere
- Pressure = density X gas constant X temperature
- If any two are known the third can be determined
- Relationship between any two depends on third
- Temperature and density inversely related at same pressure (height)
- Colder air at lower pressure (higher height) can be less dense than warmer air at higher pressure (lower height)